Anode for cathodic protection and method for manufacturing the same

ABSTRACT

It is described a metal anode for cathodic protection in form of mesh ribbon having meshes whose holes are of rhomboidal shape, characterised by having such holes of rhomboidal shape arranged with the major diagonal oriented along the direction of the ribbon length and by the fact that the side edges along the ribbon length are free from cutting protrusions. It is also described a method for obtaining such anode.

FIELD OF THE INVENTION

The invention relates to the field of cathodic protection of reinforcedconcrete structures, and in particular to a design of anode particularlyefficient in terms of electrical resistance per unit length and offlexibility, and particularly safe to install and handle.

The invention also relates to the method of production of such anode.

BACKGROUND OF THE INVENTION

Corrosion phenomena affecting reinforced concrete structures are wellknown in the art. The steel reinforcement inserted in the cementitiousstructures to improve the mechanical properties thereof normally worksin a passivation regime induced by the alkaline concrete environment;however, after some time, the ion migration across the porous surface ofthe concrete causes a localised attack to the protective passivationfilm. Particularly worrying is the attack by chlorides, which arevirtually present in all kinds of environments where the reinforcedconcrete structures are employed, and to an even higher extent where anexposure to brackish water (bridges, pillars, buildings located inmarine zones), antifreeze salts (bridges and road structures in coldclimate zones) or even seawater, such as for instance in the case ofpiers and docks, takes place. The critical value of chloride exposurehas been esteemed around 0.6 kg per cubic metre of concrete, beyondwhich the passivation state of the reinforcing steel is not guaranteed.Another form of concrete decay is represented by the carbonatationphenomenon, that is the formation of calcium carbonate by reaction ofthe lime of the cementitious mixture with atmospheric carbon dioxide.Calcium carbonate lowers the concrete alkali content (from pH 13.5 to pH9) bringing iron in an unprotected state. The presence of chlorides andthe simultaneous carbonatation represents the worst of conditions forthe preservation of the reinforcing bar of the structures. The corrosionproducts of steel are more voluminous than steel itself, and themechanical stress resulting from their formation may lead to concretedelamination and fracturing phenomena, which translate into huge damagesfrom the point of view of economics besides the one of safety. For thisreason, it is known in the art that the most effective method forindefinitely prolonging the lifetime of reinforced concrete structuresexposed to the atmospheric agents, even in the case of relevant saltconcentrations, consists of cathodically polarising the steelreinforcement. In this way, the latter becomes the site of an oxygencathodic reduction, suppressing the anodic corrosion and dissolutionreactions. Such system, known as cathodic protection of reinforcedconcrete, is practised by coupling anodic structures of various kinds tothe concrete, respect to which the reinforcement to be protected acts asa cathodic counterelectrode; the electrical currents involved supportedby an external rectifier transit across the electrolyte consisting ofthe porous concrete partially soaked with a salty solution.

The anodes commonly used for the cathodic protection of reinforcedconcrete consist of a titanium substrate coated with transition metaloxides or other types of catalysts for anodic oxygen evolution. As thesubstrate it is possible to make use of other valve metals, either pureor alloyed; pure titanium is however the largely preferred choice forthe sake of cost.

European Patent EP458951 discloses a grid-type electrodic structure forcathodic protection consisting of a plurality of metal ribbons having anelectrocatalytic coating, said metal ribbons having voids of differentgeometries.

This type of ribbons can be manufactured by punching of solid metalribbons or more commonly by the traditional methods of metal expansionwherein a metal sheet is expanded by pressuring and punching through aseries of knives arranged orthogonal to the advancement direction of theribbon itself. This first step allows obtaining an expanded metal sheet.Such sheet is then subjected to a second step of cutting suitable forobtaining ribbons of the required dimensions. Said expanded metalribbons present meshes having voids of rhomboidal shape with the majordiagonal oriented orthogonal to the ribbon length.

This method of manufacturing has the inconvenience of producing metalribbons with meshes having cutting side protrusions automatically formedduring the operation of cutting, making these anodes difficult to handleand the installation phase accordingly dangerous.

Metal ribbons with smooth lateral edges are disclosed in Canadian PatentApplication CA 2078616 A1; by the method described this document, theribbons obtained present a continuous longitudinally-extending solidsection of a certain width, which is invariably formed in themanufacturing process and which can only be used for spot-welding. Inpresent-day cathodic protection systems, however, it is preferred not toweld ribbon anodes at all, but rather to overlay them directly to thereinforcement with plastic spacers arranged in-between. In such case,the longitudinally-extending solid section is just a loss of material,especially because this solid section invariably gets coated withprecious metals during the application of the catalytic layer. Suchcatalytic layer however cannot work properly on a non-foraminousstructure and affects the calculation of the actual current densityimpressed to the anodic structure, thereby complicating the design ofthe overall cathodic protection system.

SUMMARY OF THE INVENTION

Various aspects of the invention are set out in the accompanying claims.

Under one aspect, the invention relates to an anode in form of meshribbon for systems of cathodic protection, for instance of cathodicprotection of reinforced concrete structures, overcoming theinconveniences of the prior art, whose edges are substantially free ofdiscontinuities in form of cutting protrusions and have a sinusoidalshape.

In the context of the present description reference is made, for thesake of simplicity, to cathodic protection of reinforced concretestructures; it is understood that the invention may be practised in thefield of cathodic protection in general, for instance comprising thecathodic protection of metal tank bottoms.

Under another aspect, the invention relates to a method formanufacturing said anode.

Under a further aspect, the invention relates to a cathodic protectionsystem comprising at least one anode in form of mesh ribbon whose edgesare substantially free of cutting protrusions.

Some of the most significant results obtained by the inventors arepresented in the following description, which is merely provided by wayof example without wishing to limit the invention.

The anode according to the invention consists of a ribbon of expandedmetal characterised by meshes with rhomboidal shaped voids with themajor diagonal oriented along the direction of the ribbon length. In oneembodiment, the lateral edges of the ribbon have a sinusoidal profileand are free of cutting protrusions.

The inventors have surprisingly noticed that an anode for cathodicprotection as hereinbefore described displays a remarkably reduced ohmicresistance per unit length, for instance up to 4-fold reduced, withrespect to the anodes of the prior art.

The lower electrical resistance makes possible to reduce the number ofelectrical connections, for instance in a grid system, with sensiblesavings of material and installation time.

In one embodiment, the metal mesh ribbon is made of titanium.

In another embodiment, the metal mesh ribbon is coated with a catalyticcoating containing noble metals or oxides thereof.

In one embodiment, the dimensions of the ribbon can have a width rangingfrom 3 mm to 100 mm with a thickness of 0.25 mm to 2.5 mm and a lengthof 1 m to 150 m.

BRIEF DESCRIPTION OF THE DRAWINGS

For the sake of a better understanding of the invention, reference willbe made to the following drawings, having the purpose of depicting somepreferred embodiments thereof without limiting its extent.

FIG. 1A shows a top-view of a traditional expanded metal anode.

FIG. 1B shows a top-view of an expanded metal anode according to theinvention.

DETAILED DESCRIPTION OF THE DRAWINGS

In detail, FIG. 1A shows a top view of the traditional anode in whichare distinguishable cutting protrusions 1 due to the manufacturingmethod including a cutting step, the rhomboidal geometry with majordiagonal 3 of rhomboidal voids arranged in the direction of the ribbonwidth and the minor diagonal 4 of the same arranged in the direction ofthe ribbon length.

FIG. 1B shows a top-view of the anode according to the invention inwhich are distinguishable non-cutting blunt lateral edges 2, therhomboidal geometry with major diagonal 3 of rhomboidal voids arrangedin the direction of the ribbon length and the minor diagonal 4 of thesame arranged in the direction of the ribbon width.

EXAMPLE

Some of the most significant results obtained by the inventors arereported in Table 1, wherein ohmic resistance data of representativeanodes of the invention are compared to traditional anodes. Anodeslabelled A and B are anodes of rhomboidal geometry with the majordiagonal of rhomboids oriented orthogonal to the ribbon length likewisedepicted in FIG. 1A, traditionally obtained by longitudinal expansionwith respect to the displacement direction of a solid metal ribbon.Anodes labelled C and D are anodes of rhomboidal geometry according toone embodiment of the invention, likewise depicted in FIG. 1B.

Anodes C and D were prepared by orthogonal expansion with respect to thedisplacement direction of a solid metal ribbon allowed to run in anapparatus along a parallel row of knives which expand the solid ribbonin an orthogonal direction by pressuring and punching. The ribbonmanufacturing is completed by means of a last series of knives, havingblades of predefined length higher than the blades of previous knives,which upon applying a pressure are suitable for modelling the lateraledge of the ribbon as depicted in FIG. 1B. Besides the advantagesalready explained in terms of conductivity due to the anode geometry,this method has the advantage of providing an expanded metal ribbon freeof longitudinally-extending solid sections which, being not subsequentlycut, does not present any cutting edge and is therefore much safer andeasy to handle during the installation. This method moreover allows toadvantageously obtain a metal ribbon of the desired length directly uponcompletion of the expansion. Such method of production furthermoreallows obtaining ribbons of higher length than the traditional methodthereby facilitating big size installation which would requireconnections of multiple ribbons, with a lower solidity of the overallanodic system.

From the data reported in the table it can be noticed that for a givenwidth, the anodes of the invention display an ohmic resistance about 60%lower.

TABLE 1 R-Ohmic Resistance Anodes in accordance with FIG. 1A A—20 mmwide  0.22 Ohm/m B—10 mm wide  0.43 Ohm/m Anodes in accordance with FIG.1B C—20 mm wide 0.088 Ohm/m D—10 mm wide 0.177 Ohm/m

The previous description is not intended to limit the invention, whichmay be used according to different embodiments without departing fromthe scopes thereof, and whose extent is univocally defined by theappended claims.

Throughout the description and claims of the present application, theterm “comprise” and variations thereof such as “comprising” and“comprises” are not intended to exclude the presence of other elementsor additives.

The discussion of documents, acts, materials, devices, articles and thelike is included in this specification solely for the purpose ofproviding a context for the present invention. It is not suggested orrepresented that any or all of these matters formed part of the priorart base or were common general knowledge in the field relevant to thepresent invention before the priority date of each claim of thisapplication.

We claim:
 1. Anode for cathodic protection comprising an expanded metalribbon with rhomboidal meshes free of longitudinally-extending solidsections, wherein said rhomboidal meshes are geometrically arranged withthe major diagonal parallel to the direction of the ribbon length. 2.Anode according to claim 1, wherein the lateral edge profiles along thelength of said ribbon are free of discontinuities.
 3. Anode according toclaim 1, wherein said metal is titanium.
 4. Anode according to claim 1,wherein said metal is coated with a catalytic layer.
 5. Anode accordingto claim 4, wherein said catalytic layer comprises noble metals oroxides thereof.
 6. Method of manufacturing the anode according to claim1 comprising: running of a metal ribbon through an expanding deviceequipped with at least one row of knives of a first predetermined lengtharranged parallel to the direction of the ribbon displacement, expandingof the metal ribbon by means of pressing and punching action of said atleast one row of knives, forming of lateral edge profiles of theexpanded metal ribbon by means of pressing and punching action of a lastrow of knives having blades of a second predetermined length higher thansaid first length.
 7. A cathodic protection system comprising at leastone anode according to claim 1 embedded in a cementitious structureequipped with metal reinforcement bars.
 8. Method for cathodicprotection of a reinforced concrete structure comprising applying ananodic potential to said anodes of said cathodic protection system ofclaim 7.